These experiments were the first to adapt the 3 min-all-out test to isometric contractions of a single muscle group. These experiments established that the critical torque measured using intermittent submaximal contractions performed to task failure can also be closely estimated using repeated isometric contractions leading to a plateau in torque. In addition, these experiments utilised supramaximal muscle stimulation to estimate central and peripheral contributions to muscle fatigue during all-out exercise, and established the predominance of peripheral fatigue in this situation. This work cemented the fundamental physiological relationship between all-out exercise and muscular endurance, the culmination of 5 years work. Burnley, M. (2009). Estimation of critical torque using intermittent isometric maximal voluntary contractions of the quadriceps in humans. Journal of Applied Physiology, 106, 975-983.

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To determine whether the asymptote of the torque-duration relationship (critical torque) could be estimated from the torque measured at the end of a series of maximal voluntary contractions (MVCs) of the quadriceps, eight healthy men performed eight laboratory tests. Following familiarization, subjects performed two tests in which they were required to perform 60 isometric MVCs over a period of 5 min (3 s contraction, 2 s rest), and five tests involving intermittent isometric contractions at ~35-60% MVC, each performed to task failure. Critical torque was determined using linear regression of the torque-impulse and contraction time during the submaximal tests, and the end-test torque during the MVCs was calculated from the mean of the last six contractions of the test. During the MVCs voluntary torque declined from 263.9 ± 44.6 N.m to 77.8 ± 17.8 N.m. The end-test torque was not different from the critical torque (77.9 ± 15.9 N.m; 95% paired-sample confidence interval, –6.5 to 6.2 N.m). The root mean squared error of the estimation of critical torque from the end-test torque was 7.1 N.m. Twitch interpolation showed that voluntary activation declined from 90.9 ± 6.5% to 66.9 ± 13.1% (P <0.001) and the potentiated doublet response declined from 97.7 ± 23.0 N.m to 46.9 ± 6.7 N.m (P <0.001) during the MVCs, indicating the development of both central and peripheral fatigue. These data indicate that fatigue during 5 min of intermittent isometric MVCs of the quadriceps leads to an end-test torque that closely approximates the critical torque.

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eng

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Journal of Applied Physiology

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dc.title

Estimation of critical torque using intermittent isometric maximal voluntary contractions of the quadriceps in humans